Polymers having chemicophysically-tailored surfaces have broad potential uses, particularly in the areas of separations and electronic and biomedical devices. For example, in the biomedical field, surface-modified polymers are potentially useful as implantable devices that exhibit hydroxyapatite-like surfaces. Such hydroxyapatite-like surfaces have the unique ability to support osteointegration with bone tissues.
Total joint replacement has enjoyed substantial success in orthopedic surgery. Two methods are currently employed for fixating the load-bearing implants within bone tissue. Those methods include: (1) the use of grouting materials such as poly(methyl methacrylate) (PMMA) as bone cement between the bone and the prosthesis; and (2) direct opposition of bone tissue onto porous and non-porous implant surfaces. The latter method is known as the "cementless hip replacement" and has been gaining in clinical acceptance despite certain disadvantages.
In using the cementless implant method, a prosthesis is coated with hydroxyapatite which is a major inorganic component of bone. The hydroxyapatite-coated prosthesis is then implanted in the bone cavity. The hydroxyapatite, which is a calcium salt is believed to facilitate osteointegration with the bone tissues. After partial integration of the hydroxyapatite-coated prosthesis with the bone, layers of hydroxyapatite can be detected between the prosthesis and the bone tissues.
Despite the success of metal femoral components in many patients, long term data has demonstrated an unacceptably high failure rate in more active patients due to loosening of the femoral stem caused by bone resorption around the implant. Bone resorption results from stress shielding of the bone around the implant due to the high modulus of the metal stem coupled with unstable fixation of the stem in the surrounding hard tissue.
The failure to achieve bone ingrowth into the surface of the implant to support implant mechanical stability has been a major problem with cementless replacements. Surfaces of the currently-employed metallic implants have been treated with hydroxyapatite in an attempt to overcome this problem and induce bone ingrowth about the prosthesis as described above. Mechanical tolerances of the implant-bone interface have been improved somewhat, but the mechanical stability of the interface still suffers from dislodgement of the prosthesis from the bone cavity. Further aggravation occurs due to the stiffness mismatch of the implant and bone which leads to bone resorption.
Interest in high-performance thermoplastic load-bearing implants has grown significantly over the past few years. Because bone tissue itself is a composite material composed of hydroxyapatite ceramic-reinforced collagen, polymeric matrix composites are believed to be capable of remedying problems associated with metal stems. Benefits of a composite prosthetic material include a relatively low elastic modulus compared to current implant metals, the absence of metal ion releases in the body, and the ability to customize the strength required for the implant to best suit a particular design requirement. Non-metallic composite prostheses are generally lightweight and outperform the metallic prostheses in terms of strength and stability on a per mass basis. Health problems created by potentially carcinogenic metallic alloys can also avoided when non-metallic, generally ceramic composite, prostheses are employed.
Of particular interest as polymeric implants are the polyetheretherketone (PEEK) fiber-reinforced composites such as those entailing carbon fibers. PEEK exhibits considerable chemical resistance, toughness, and excellent mechanical properties. Unfortunately, the stabilization of these composite polymeric prostheses in the surrounding hard bone tissues has not proved adequate for prolonged use of these implants. The lack of biological incentive for bone formation at the uppermost surface of these implants has resulted in the failure or limited realization of osteointegration.
Thus far, no effective method of providing a high-strength, permanently stable interface between non-metallic prostheses and bone tissue has been developed. By way of the present invention, it is speculated that formation of direct chemical bonds between organic polymeric surfaces and the inorganic hydroxyapatite may provide the maximum attraction forces. Of the few known forms of direct bonding of two dissimilar substrates, silicon coupling agents have been recognized as providing adhesion where chemical bonding was thought to exist.
The lack of a high-bonding coating for metal or non-metallic implants has prevented the development of a prosthesis that maintains its initial mechanical stability for the extended period of normal usage. The strength of the interface between the prosthesis and the hydroxyapatite coating often weakens as the implant patient grows older and the aggregate amount of wear and tear on the interface increases.
The surface-phosphonylated polymers of the present invention have not heretofore been employed at the interface between hydroxyapatite in the bone tissue and the prostheses. Chain or mass phosphonylated polymers have, however, been known.
Phosphonylation involves the attachment of a --P(O)Cl.sub.2 group onto a substrate. Organic polyphosphonates made by the direct polymerization of substituted vinyl monomers have been evaluated as potential preventive agents for dental carriers and as adhesives useful for the restoration of teeth. Specifically, phosphonates are known to adhere to dentin. Furthermore, phosphonylated polyethylene solutions have been studied relative to their adsorption to enamel.
Various patents have been directed to the mass phosphonylation of polymers. Generally, such mass phosphonylation achieves a random dispersion of the phosphonylates throughout the entire polymer mass by phosphonylating the polymer in solution with phosphorus trichloride and oxygen.
For example, U.S. Pat. No. 3,097,194 to Leonard is directed to a process for preparing elastomeric phosphorylated amorphous copolymers of ethylene and propylene which are essentially free of low molecular weight polymer oils. Phosphorylation, or esterification of the copolymer, is conducted in situ in the copolymer solution mass by inactivating a polymerization catalyst with water and oxygen to convert the catalyst to an inert metal oxide. Oxygen is then bubbled through the reaction mass in the presence of phosphorus trichloride to obtain the phosphorylated copolymer.
An example of phosphonated polymers is provided in U.S. Pat. No. 3,278,464 to Boyer et al. In accordance therewith, ethylenically unsaturated polymers are reacted with an organic-substituted phosphorus compound to produce phosphonated polymers. Like the process described in the preceding paragraph, attachment of the phosphorus groups results in near-homogeneous, or mass, phosphonylation wherein the polymer and phosphorus compound are combined in a solvent system.
Moreover, in U.S. Pat. No. 4,207,405 to Masler et al., polyphosphates are provided that are the homogenous reaction products, in an organic solvent, of phosphorus acid or phosphorus trichloride and a water-soluble carboxyl polymer. U.S. Pat. Nos. 3,069,372 to Schroeder et al., 4,678,840 to Fong et al., 4,774,262 to Blanquet et al., 4,581,415 to Boyle, Jr. et al., and 4,500,684 to Tucker show various phosphorus-containing polymer compounds.
U.S. Patents Nos. 4,814,423 and 4,966,934 to Huang et al. describe adhesives for bonding polymeric materials to the collagen and calcium of teeth. For bonding to calcium, the adhesive employs an ethylenically unsaturated polymeric monophosphate component. A tooth is coated with the adhesive and then a filling is applied.
Although various phosphonylated polymers are known, the particular features of the present invention are absent from the art. The prior art is generally deficient in affording a preformed solid polymer having phosphorus-containing groups covalently attached to only its surface or near-surface, carbons. By providing an organic polymer preform having surface phosphonylation, the present invention allows high-strength organic polymer prostheses with custom-tailored stiffness to be employed as orthopedic implants. The hydroxyapatite-like phosphonylated surfaces on the preformed polymers may bind chemically to the hydroxyapatite found in bone tissue to form durable and strong implant/bone interfaces. Although primarily useful in the orthopedic implant field, the surface-phosphonylated polymers of the present invention also have potential applications in controlled drug delivery and attachment of cells such as fibroblast. For infectious microorganisms associated with device-centered infections, phosphonylated surfaces can provide controlled interaction with implantable devices. Other applications include the use of phosphonylated surfaces in the development of chromatographic, electronic, and conductive devices, and sensors and devices for isolation and/or purification of important proteins such as growth factors.